The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
Active materials, including shape memory alloy (SMA) materials are compositions that exhibit a change in material properties, e.g., stiffness, shape, and/or dimension in response to an activation signal. An activation signal can include one or more of electrical, magnetic, thermal, and other signals, and can be passively or actively communicated to an active material to effect a change in the material property.
Shape memory alloy (SMA) materials refer to a group of metallic materials that undergo a reversible change in a characteristic property when activated by an external stimulus, including an ability to return to a previously defined shape or dimension when subjected to an activation signal, e.g., a thermal activation signal.
SMA materials undergo phase transitions leading to changes in yield strength, stiffness, dimension, and shape in response to temperature. SMA materials can exist in several different temperature-dependent phases, including martensite and austenite phases. The martensite phase refers to a more deformable and less stiff phase that occurs at lower material temperatures. The austenite phase refers to a stiffer and more rigid phase that occurs at higher material temperatures. There are transformation temperature ranges including start temperatures and end temperatures over which a shape memory alloy transforms between the martensite and austenite phases. An SMA material in the martensite phase changes into the austenite phase over an austenite transformation temperature range with increasing material temperature. An SMA material in the austenite phase changes into the martensite phase over a martensite transformation temperature range with decreasing temperature. A shape memory alloy has a lower modulus of elasticity in the martensite phase and has a higher modulus of elasticity in the austenite phase.
SMA materials can include metal alloys including platinum-group metals. Known SMA materials also include certain copper alloys (CuAlZn) and nickel-titanium-based alloys, such as near-equiatomic NiTi, known as Nitinol and some ternary alloys such as NiTiCu and NiTiNb. SMA materials including NiTi can withstand large stresses and can recover strains near 8% for low cycle use or up to about 2.5% for high cycle use.
SMA material properties include large recoverable strains due to crystallographic transformations between the martensite and austenite phases. As a result, SMA materials can provide large reversible shape changes or large force generation. SMA material behavior is due to a reversible thermoelastic crystalline phase transformation between a high symmetry parent phase, i.e., austenite phase, and a low symmetry product phase, i.e., martensite phase. The phase changes between the austenite and martensite phases occur as a result of changes in either one of stress and temperature.
Known methods for controlling activation of SMA materials include mechanical-based devices including a micro-switch. Known micro-switches have poor control associated with on/off control strategies that are based on ending position of the actuator. An overload protection mechanism is often employed to combat the poor controllability of a micro switch, which adds to cost, size and complexity.